The most immediate application of the present invention is with Multi-Chip Modules, and that type of electronic semiconductor component is considered first. As is known, Multi-Chip Modules ("MCMs") perform a variety of electronic functions, and find ever increasing usage in sophisticated electronic applications, particularly airborne and spaceborne. By definition, an MCM contains two or more semiconductor die or chips, as variously termed, and ancillary electrical components, assembled in a single enclosed package, that together comprise an electronic circuit function. The semiconductor chips contain the micro-miniature integrated circuits, such as processors, amplifiers, memory, and the like. In one type of MCM structure, the semiconductor chips and components are supported on a common base, consisting of an integral multi-layer printed wiring structure, referred to as a "substrate". Often that substrate is formed of ceramic, an electrical insulator that is rigid, allows for plated-on conductors of the finest widths and spacing with the greatest accuracy and is able to maintain a hermetic seal. Metallic conductors printed on various layers of the substrate, and metallic vias through the layers, serve to electrically connect the semiconductor chips to each other and to the MCM's external interfaces.
Completing the module, the foregoing elements are contained together in a single enclosed package, often hermetically sealed, that serves as a protective housing for the semiconductor chips and associated components. The ceramic substrate, being hermetic, serves as the bottom wall to the module. A metal wall, or sealring, is brazed to the substrate around the perimeter, encompassing the components. A lid welded to the top surface of this sealring hermetically seals the components inside. A number of leads extend out the four sides of the module to provide external electrical input-output connections to the MCM.
The foregoing is a brief introduction to those known devices for the benefit of lay readers, and, for additional details of their construction, the interested reader may make reference to the technical literature.
In practice such MCMs are generally installed upon a larger circuit board which contains the electrical interconnections between the MCMs and other components thereon. This circuit board is typically constructed of a material such as glass-epoxy or glass-polyimide, a less expensive and less quality material than the ceramic.
For airborne and space applications, MCMs are typically bonded to the circuit boards. Bonding enhances thermal conductivity to the MCM, and isolates mechanical loads from the MCM's input-output connections, which promotes longer product life. Such bonding can be accomplished with a variety of adhesives, such as thermosetting epoxies or thermoplastics, and solder.
To bond the MCM in place, as example, a layer of thermally sensitive adhesive is applied to either the underside surface of the MCM, or directly to the surface of the circuit board at the location to which that component is to be placed. With the MCMs and all other components for that circuit board properly positioned, the board is then placed in an oven and the temperature raised to cure or reflow the adhesive, attaching the MCMs and other components in place. When removed from the oven and cooled down to room temperature the MCM's are firmly assembled to the circuit board.
If failed components are detected during subsequent electrical testing of the assembled board, they must be removed from the board for repair or replacement. This is traditionally accomplished by locally re-heating the adhesive with a stream of hot air from a heat gun or specialized nozzle, raising the temperature sufficiently to soften or weaken the adhesive bond, yet not to so high a temperature as to delaminate or damage the layers that form the circuit board or to cause damage to the component being removed. The component may then be easily twisted or pried off the surface, typically leaving residual amounts of adhesive which may be scrubbed or chemically cleaned from the circuit board's surface.
For small MCMs or other components, the forgoing detachment procedure is practical and commonly employed. However, for large MCMs or components, meaning those approximately 1.5 inches on a side or greater, the foregoing procedure proves inadequate.
Typical adhesives possess relatively low thermal conductivities. When heat is applied to one edge of a thin layer of such an adhesive at an elevated temperature and allowed to flow along the layer, one finds that, due to the material's low thermal conductivity, the temperature at different positions along the layer is significantly lower the farther the position is from the heated edge. Should one wish to raise the temperature at the center of that adhesive layer to a prescribed temperature, the temperature of the heated edge must be raised to a much higher level to compensate for the temperature drop along the thermal path through the adhesive to that center region of the adhesive layer.
Because of that high resistance heat path from the peripheral edges of the MCM or other large component to the central region of the bonding adhesive, adequate temperature elevation is difficult to achieve in the central region without driving outer portions of the MCM and circuit board to prohibitively high temperatures. Compounding that difficulty is the fact that the circuit board itself is an effective heat sink.
Circuit boards are designed to efficiently draw heat away from MCMs and other components during normal operation by use of such means as thermal vias and planar metalization patterns. Thus a circuit board also effectively draws heat away from an MCM or other component, and its underlying adhesive during the detachment process. Depending on the size of the MCM or other component, and the efficiency of the circuit board's thermal features, it may in fact be impossible to drive sufficient heat into the adhesive bondline at its perimeter to raise the temperature at its center to a sufficient temperature for removal.
Directing hot air at the top of the MCM instead, to drive heat vertically down through the MCM, is not a viable alternative. Since the module's substrate is separated from the module's cover by a space filled with an inert sealing gas, inherently a thermal insulator, that alternative heat path possesses an even greater thermal resistance than that from the edges.
Pulling or shearing the MCM off of the circuit board without adequate heating in the central region creates large physical stresses in both the MCM and the circuit board. Because of these high stresses, the MCM is often destroyed during removal. Since MCMs contain multiple integrated circuits, it is generally cost effective to rework a failed MCM by removing and replacing the one failed semiconductor die or other component within the MCM, rather than discarding and replacing the entire MCM. Thus it is desirable to detach a failed MCM-from its circuit board intact, such that it can be repaired and returned to the circuit board. Furthermore, the high stresses caused by pulling an MCM from the circuit board without adequate heating also places the circuit board at risk of damage. If the board itself is damaged, the entire assembly must be discarded, at great cost.
Solder is another known thermally sensitive "adhesive" material used to fasten parts together. A second known technique for fastening the MCM to the circuit board is the solder ball grid array. Instead of incorporating electrical leads extending from the side of the MCM package and using a separate adhesive for fastening the MCM to the circuit board, as in the foregoing structure, the electrical leads are instead formed by electrical vias extending through the multiple layers of substrate to the underside surface of the MCM package. At the underside the terminal end of those vias typically appear by design arranged in regular rows and columns. Minute solder balls or solder columns, different geometries for the dab of solder collectively referred to herein as solder balls, are formed at the terminal ends of those vias on the underside of the substrate.
The MCM package is placed upon the circuit board, the latter of which contains solder pads that mate with the solder balls on the MCM package and the temperature is raised above the solder eutectic at which the solder reflows. When cooled, the solder solidifies and provides a firm mechanical connection that fastens the MCM package to the circuit board as well as completing the electrical connections to printed circuitry on the circuit board. The foregoing connection apparatus and technique is well known.
For removal of that solder ball grid array package from the circuit board is by the same described rework technique for the adhesively fastened MCM's. That is, sufficient heat is applied to the soldered connections to reflow, that is, liquify, the solder. And the same difficulty is evident where the MCM package is of a large size making it difficult to apply sufficient heat to the central region of the underside of the MCM package. The present invention also resolves that difficulty.
The foregoing need for rework in MCM's was earlier recognized and addressed in U.S. Pat. No. 5,624,750, granted April 1997 to Martinez et al, entitled "Adhesive Heater and Method for Securing an Object to a Surface", hereafter referred to as the "Martinez patent". The Martinez patent proposed the installation of a triple-layer laminate in between the MCM and the circuit board, in which the two outer layers comprised a thermoplastic adhesive, encapsulating an electrical heater element as the intermediate layer. The heater element therein is a resistive metal foil formed into a single serpentine pattern.
To mount the MCM to the circuit board, electrical current heated and softened the thermoplastic layers, and, with pressure simultaneously applied to the top of the MCM as one would do with a thermoplastic material, when the thermoplastic material cured, the MCM was adhesively secured in place to the PWB. The heater was left sandwiched in the adhesive, with its terminals exposed.
According to the Martinez patent, when it became necessary to remove the MCM from the circuit board for rework, a current was reapplied to the heater, and the heat thereby generated uniformly across the adhesive interface was sufficient to soften the adhesive bond holding the MCM in place. The MCM and heater would then be removed without damage to the circuit board or adjacent components. A repaired or replacement MCM could then be secured back into the original position by a like arrangement.
Although the Martinez patent's solution is interesting, upon consideration one recognizes that the resulting assembly process is more cumbersome than standard processes, and leaves extra elements in the assembly which are unnecessary during normal operation.
Also, the layered adhesive sandwich in the Martinez solution may also be thicker than the traditional adhesive bond joint, and this thickness represents additional thermal resistance between the MCM and circuit board during normal operation, resulting in higher operating temperatures for the chips within the MCM, and ultimately lower reliability or shorter product life.
Furthermore, this embedded foil heater described in the Martinez patent precludes the use of metal-filled adhesives. Adhesives loaded with metallic particles, generally silver, are commonly used to attach MCMs or other components with high operational power dissipations to a circuit board, because silver particles significantly improve the thermal conductivity of the adhesive, thus lowering the operating temperatures of these high power dissipation components. Metallic-filled adhesives are also used when it is necessary to establish an electrically conductive path from the back side of an MCM or other component to the circuit board, as is sometimes required for proper electrical operation.
An embedded foil heater would be incompatible with a metal-filled adhesive, as the conductive particles would short circuit the serpentine pattern and make it unsuitable as a heater element. Martinez's approach would also appear incompatible with a solder ball grid array, which the Martinez patent does not address. The structure proposed by the Martinez patent is not known to applicants to be in industrial practice and evident disabilities in that structure may be a reason why.
Like the structure in the Martinez patent, the present invention also makes use of an electrical heater to solve the problem of detaching an MCM for rework. However, as an advantage, the invention does not add extra elements to the traditional circuit board assembly process, does not significantly increase the thermal resistance of the path from the integrated circuits within the MCM to the circuit board and its accompanying heat sink, and does not preclude the use of thermally and electrically conductive metal-filled adhesives.
The problem in reworking MCM's, whether fastened to the circuit board by regular adhesives or with a solder ball grid array, is recognized as endemic to other large size electronic semiconductor components as well, even those that contain only a single physically large semiconductor chip. As those skilled in the art recognize, the more modern semiconductor chips are growing in physical size as more and more circuit functions are expected to be packed within a single die even in commercial devices, such as cellular telephones. As a consequence large numbers of very fine closely spaced wires are required to interface to the semiconductor die. Because the wires must all extend into the die they are necessarily physically small in width and must be packed closely together, typically one mil in diameter separated by a two mil space. However, conventional printed circuit board technology typically provides semiconductor die interface connections with no less than a four mil separation.
To resolve the apparent physical incompatibility in spacing requirements, the approach taken has been to mount the semiconductor chip onto an intermediate "interposer" substrate, which is often formed of ceramic material. The printed wiring formed on the substrate fans out from the microscopic spacings at the location of the semiconductor die or chip to the wider spacings and wider wiring required by the conventional printed circuit board.
That electronic semiconductor assembly is then mounted onto the printed circuit board. The electrical leads from the assembly's substrate are soldered to the mating solder pads on the printed circuit board, or, should the substrate instead employ a solder ball grid array, the solder balls are soldered to the mating solder pads formed on the printed circuit board. As in the case of the earlier described MCM's, in the foregoing arrangement, viewed in a generic sense, one multi-layer printed circuit board is mounted atop another printed circuit board.
The dimension critical wire bonding of the chip's electrical leads, thus, is accomplished on the ceramic substrate. Interconnect to the printed circuit board is accomplished by soldering the electrical leads from the substrate to mating pads on the conventional circuit board. With such an interposer or intermediate substrate, in retrospect, one recognizes the parallel between the foregoing structure and that of the MCM, earlier described.
Accordingly, an object of the present invention is to improve the process of reworking MCM's and other electronic semiconductor components.
A further object of the invention is to provide an improved structure for MCM's and other electronic semiconductor components that assists in detaching such MCM's or other electronic semiconductor components from its assembled position upon a circuit board when required for rework.
And a still further object of the invention is to structurally modify an MCM or other electronic semiconductor component to render it more easily removable from the surface of a circuit board for rework, without requiring the use of prohibitively high temperatures and without damaging the MCM or other component being detached, or other MCMs and components on the circuit board, or the circuit board itself.